Filtration media for filtering particulate material from gas streams

ABSTRACT

A filtration medium is disclosed for use in air filters used in heating, ventilating and air conditioning systems. The medium contains at least one nanofiber layer of fibers having diameters less than 1 μm and at least one carrier layer, each nanofiber layer having a basis weight of at least about 2.5 g/m 2 , and up to about 25 g/m 2 . The medium has sufficient stiffness to be formed into a pleated configuration.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to air filtration media, for filteringparticulate material from gas streams.

2. Description of the Related Art

Filter media typically utilized for HVAC air filters that perform atefficiencies less than 99.97% at a 0.3 micron challenge are eitherglass, cellulose or polymer based. Filters made with media in thisperformance range are typically referred to as “ASHRAE filters” sincethe American Society of Heating, Refrigerating and Air-ConditioningEngineers writes standards for the performance of filter media in suchapplications. Polymer based filter media are typically spunbond ormeltblown nonwovens that are often electrostatically enhanced to providehigher filtration efficiency at lower pressure drop when compared toglass or cellulose media manufactured by a wet laid paper-makingprocess.

Electrostatically enhanced air filter media and media manufactured bythe wet laid process, more specifically with the use of glass fibers,currently have limitations. Electrostatically treated meltblown filtermedia, as described in U.S. Pat. Nos. 4,874,659 and 4,178,157, performwell initially, but quickly lose filtration efficiency in use due todust loading as the media begin to capture particles and theelectrostatic charge thus becomes insulated. In addition, as theeffective capture of particulates is based on the electrical charge, theperformance of such filters is greatly influenced by air humidity,causing charge dissipation.

Filtration media utilizing microglass fibers and blends containingmicroglass fibers typically contain small diameter glass fibers arrangedin either a woven or nonwoven structure, having substantial resistanceto chemical attack and relatively small porosity. Such glass fiber mediaare disclosed in the following U.S. patents: Smith et al., U.S. Pat. No.2,797,163; Waggoner, U.S. Pat. No. 3,228,825; Raczek, U.S. Pat. No.3,240,663; Young et al., U.S. Pat. No. 3,249,491; Bodendorf et al., U.S.Pat. No. 3,253,978; Adams, U.S. Pat. No. 3,375,155; and Pews et al.,U.S. Pat. No. 3,882,135. Microglass fibers and blends containingmicroglass fibers are typically relatively brittle and can break whenpleated, and produce undesirable yield losses. Broken microglass fiberscan also be released into the air by filters containing microglassfibers, creating a potential health hazard if the microglass were to beinhaled.

It would be desirable to provide a means for achieving ASHRAE level airfiltration while avoiding the above-listed limitations of knownfiltration media.

SUMMARY OF THE INVENTION

In a first embodiment, the present invention is directed to a filtrationmedium comprising at least one nanofiber layer of continuous polymericfibers having diameters less than about 1000 nanometers, each nanofiberlayer having a basis weight of at least about 2.5 g/m², and at least onescrim layer, wherein the medium has a filtration efficiency of at leastabout 20% when challenged with particles having an average diameter of0.3 μm in air flowing at a face velocity of 5.33 cm/sec, and aHandle-o-meter stiffness of at least about 45 g.

A second embodiment of the present invention is directed to a processfor filtering particulate matter from an air stream comprising passingan air stream containing particulate matter through a filtration mediumcomprising at least one nanofiber layer of continuous polymeric fibersand at least one scrim layer, wherein the continuous polymeric fibers ofthe nanofiber layer have diameters less than about 1000 nanometers andwherein each nanofiber layer has a basis weight of at least about 2.5g/m² and a thickness of less than about 100 μm, and wherein thefiltration medium has a Handle-o-meter stiffness of at least about 45 g,and filtering up to about 99.97% of particles having an average diameterof 0.3 μm in an air stream moving at a face velocity of 5.33 cm/sec.

Another embodiment of the present invention is directed to a process offorming a filtration medium comprising providing at least one scrimlayer having a Handle-o-meter stiffness of at least about 10 g on amoving collection belt, and depositing nanofibers on the scrim layer toform a single nanofiber layer having a basis weight of at least about2.5 g/m² to form a filtration medium having a Handle-o-meter stiffnessof at least about 10 g and a pressure drop of less than about 30 mm H₂O.

DEFINITIONS

The term “nanofibers” refers to fibers having diameters of less than1,000 nanometers.

The term “filter media” or “media” refers to a material or collection ofmaterials through which a particulate-carrying fluid passes, with aconcomitant and at least temporary deposition of the particulatematerial in or on the media.

The term “ASHRAE filter” refers to any filter suitable for use inheating, ventilation and air conditioning systems for filteringparticles from air.

The term “SN structure” refers to a multilayer nonwoven materialcontaining a spunbond (S) layer and a nanofiber (N) layer.

The term “SNS structure” refers to a multilayer nonwoven materialcontaining a nanofiber layer sandwiched between two spunbond layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a prior art electroblowing apparatus forforming nanofibers suitable for use in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a filter medium comprising at least onenanofiber layer and at least one scrim layer. The nanofiber layercomprises a collection of substantially continuous organic polymericnanofibers in a filtration medium layer, the nanofibers having diametersless than about 1 μm or 1000 nm. Such filter media can be used infiltration applications for removing particulate material from a fluidstream, in particular, particulate material from a gaseous stream suchas air.

Filtration media suitable for use in air filtration applications,including ASHRAE filtration and vehicle cabin air filtration, can bemade by layering one or more nanofiber layer(s) with a scrim layer toform an SN_(x) structure, or by sandwiching one or more nanofiber layersbetween two scrim layers to form a SN_(x)S structure, where x is atleast one. Each nanofiber layer has a basis weight of at least about 2.5g/m², and the total basis weight of the nanofiber layers is about 25g/m². Additionally, the filter medium can contain other layers such asone or more meltblown (M) layers.

In the medium of the invention, the nanofiber layer has a thickness ofless than about 100 μm; advantageously the thickness of the nanofiberlayer is greater than 5 μm and less than 100 μm. The thickness of thenanofiber layer can vary depending on the density of the nanofiberpolymer. The thickness of the nanofiber layer can be reduced withoutsubstantial reduction in efficiency or other filter properties if thesolids volume fraction of the nanofiber layer is increased, such as bycalendering or by collecting the nanofiber layer under high vacuum.Increasing the solidity, at constant layer thickness, reduces pore sizeand increases particulate storage.

The nanofiber layer in the present invention may be made in accordancewith the barrier webs disclosed in U.S. Published Patent Application No.2004/0116028 A1, which is incorporated herein by reference.

The nanofiber layer is made up of substantially continuous polymericfibers having diameters less than 1000 nm, advantageously between about100 nm and about 700 nm, or even more advantageously between about 300nm and about 650 nm. The continuous polymeric fibers of the nanofiberlayer can be formed by any process capable of making continuous fibersin this diameter range, including electrostatic spinning orelectroblowing. A process for forming nanofibers via electroblowing isdisclosed in PCT Patent Publication Number WO 03/080905A (correspondingto U.S. Ser. No. 10/477,882, filed Nov. 20, 2002), which is incorporatedherein by reference. WO 03/080905A discloses an apparatus and method forproducing a nanofiber web, the apparatus essentially as shown in FIG. 1.The method comprises feeding a stream of polymeric solution comprising apolymer and a solvent from a storage tank 100 to a series of spinningnozzles 104 within a spinneret 102 to which a high voltage is appliedthrough which the polymeric solution is discharged. Meanwhile,compressed air that is optionally heated in air heater 108 is issuedfrom air nozzles 106 disposed in the sides or the periphery of spinningnozzle 104. The air is directed generally downward as a blowing gasstream which envelopes and forwards the newly issued polymeric solutionand aids in the formation of the fibrous web, which is collected on agrounded porous collection belt 110 above a vacuum chamber 114, whichhas vacuum applied from the inlet of air blower 112.

The filter medium of the invention may be made by adhesively laminatingthe nanofiber layer to the carrier layer (also referred to herein as a“scrim”), or by forming the nanofiber layer directly on the carrier orscrim layer by placing the scrim layer on the collection belt 110 in theabove described process to form an SN structure, in which case thenanofiber layer is adhered to the scrim layer by mechanicalentanglement. The medium of the invention can be made by forming ananofiber layer in a single pass or by building up the nanofiber layerto the desired thickness or basis weight using multiple passes, e.g., inan electroblowing process. The electroblowing process allows a nanofiberlayer of suitable basis weight for use in an air filter medium to beformed in a single pass because a higher throughput is possible thanpreviously known in the production of nanofibers. The nanofiber layermay be formed with a collection belt speed of at least 5 m/minute, andadvantageously at least 10 m/minute. The polymer solution throughput inthe electroblowing process for forming nanofibers is at least about 1cm³/min/hole of the spinneret, and advantageously at least about 2cm³/min/hole. Therefore, by configuring the spinneret to have a seriesof spinning nozzles or holes along the length of the spinneret, anddelivering the polymer solution through each nozzle or hole at such highrates of flow, a higher basis weight nanofiber layer than known to datecan be formed on a scrim layer in a single pass. Depending on thepolymer solution flow rate and the collection belt speed, singlenanofiber layers having basis weights of between about 2.5 g/m² and evenup to 25 g/m² can be formed in a single pass. In conventional processesfor forming nanofiber-containing filtration media, forming a nanofiberlayer of suitable basis weight on a scrim requires repeated passes ofthe scrim through the nanofiber formation process to build up to a basisweight of even 1 g/m². By forming the nanofiber layer in one passaccording to the present invention, less handling is required, reducingthe opportunity for defects to be introduced in the final filter medium.The higher polymer solution throughput of the electroblowing processprovides a more economical process than previously known in theproduction of nanofibers. Of course, those skilled in the art willrecognize that under certain circumstances it can be advantageous toadjust the spinning conditions to deposit multiple nanofiber layers ofat least about 2.5 g/m² in multiple passes in order to build-up thetotal nanofiber layer basis weight to as much as about 25 g/m².Variations in the spinning conditions to modify the nanofiber laydownrate, and therefore the basis weight of a single nanofiber layer, can bemade in the collection belt speed, polymer solution flow rate and evenby varying the concentration of the polymer in the solution.

The layers of the filter medium are made from organic polymer materials.Advantageously, the scrim layers are spunbond nonwoven layers, but thescrim layers can be carded webs of nonwoven fibers and the like. Thescrim layers require sufficient stiffness to hold pleats and dead folds.The stiffness of a single scrim layer is advantageously at least 10 g,as measured by a Handle-o-meter instrument, described below.Particularly high stiffness can be achieved by using an acrylic bondedcarded or wet laid scrim comprising coarse staple fibers. Spunbondnonwovens may also be used. The filtration medium of the invention has atotal Handle-o-meter stiffness of at least 45 g. Advantageously, thefiltration medium has a structure of SNS in which at least two scrimlayers contribute to the stiffness.

The medium of the invention can be fabricated into any desired filterformat such as cartridges, flat disks, canisters, panels, bags andpouches. Within such structures, the media can be substantially pleated,rolled or otherwise positioned on support structures. The filtrationmedium of the invention can be used in virtually any conventionalstructure including flat panel filters, oval filters, cartridge filters,spiral wound filter structures and can be used in pleated, Z-filter,V-bank, or other geometric configurations involving the formation of themedium to useful shapes or profiles. Advantageous geometries includepleated and cylindrical patterns. Such cylindrical patterns aregenerally preferred because they are relatively straightforward tomanufacture, use conventional filter manufacturing techniques, and arerelatively easy to service. Pleating of media increases the mediasurface area within a given volume. Generally, major parameters withrespect to such media positioning are: pleat depth; pleat density(typically measured as a number of pleats per inch along the innerdiameter of the pleated media cylinder); and cylindrical length or pleatlength. In general, a principal factor with respect to selecting filtermedium pleat depth, pleat length, and pleat density, especially forbarrier arrangements, is the total surface area required for any givenapplication or situation. Such principles would apply, generally, to themedium of the invention and preferably to similar barrier-typearrangements.

The filter medium of the invention can be used for the removal of avariety of particulate matter from fluid streams. The particulate mattercan include both organic and inorganic contaminants. Organiccontaminants can include particulate natural products, organiccompounds, polymer particulate, food residue and other materials.Inorganic residue can include dust, metal particulate, ash, smoke, mistand other materials.

The initial pressure drop (also referred to herein as “pressure drop” or“pressure differential”) of the filter medium is advantageously lessthan about 30 mm H₂O, more advantageously less than about 24 mm H₂O. Thepressure drop across a filter increases over time during use, asparticulates plug the filter. Assuming other variables to be heldconstant, the higher the pressure drop across a filter, the shorter thefilter life. A filter typically is determined to be in need ofreplacement when a selected limiting pressure drop across the filter ismet. The limiting pressure drop varies depending on the application.Since this buildup of pressure is a result of dust (or particulate)load, for systems of equal efficiency, a longer life is typicallydirectly associated with higher load capacity. Efficiency is thepropensity of the medium to trap, rather than to pass, particulates. Ingeneral the more efficient filter media are at removing particulatesfrom a gas flow stream, the more rapidly the filter media will approachthe “lifetime” pressure differential, assuming other variables to beheld constant. The filter medium of the invention has an efficiency ofat least about 20%, meaning that the medium is capable of filtering outat least about 20% of particles having a diameter of 0.3 μm in airflowing at a face velocity of 5.33 cm/sec. For use in ASHRAE filters,advantageously, the medium of the invention is capable of filtering outat least about 30% and up to about 99.97% of 0.3 μm particles in airflowing at a face velocity of 5.33 cm/sec.

The higher the air permeability of the filter medium, the lower thepressure drop, therefore the longer the filter life, assuming othervariables are held constant. Advantageously, the Frazier airpermeability of the filter medium of the invention is at least about0.91 m³/min/m², and typically up to about 48 m³/min/m².

The filter medium of the present invention is advantageouslysubstantially electrically neutral and therefore is much less affectedby air humidity as compared with the filters disclosed in U.S. Pat. Nos.4,874,659 and 4,178,157, described above, which owe their performancesto the electrical charges associated therewith. By “substantiallyelectrically neutral” is meant that the medium does not carry adetectable electrical charge.

Test Methods

Filtration Efficiency was determined by a Fractional Efficiency FilterTester Model 3160 commercially available from TSI Incorporated (St.Paul, Minn.). The desired particle sizes of the challenge aerosolparticles were entered into the software of the tester, and the desiredfilter flow rate was set. A volumetric airflow rate of 32.4 liters/minand a face velocity of 5.33 cm/sec were used. The test continuedautomatically until the filter was challenged with every selectedparticle size. A report was then printed containing filter efficiencydata for each particle size with pressure drop.

Pressure Drop was reported by the Fractional Efficiency Filter TesterModel 3160 commercially available from TSI Incorporated (St. Paul,Minn.). The testing conditions are described under the FiltrationEfficiency test method. Pressure drop is reported in mm of water column,also referred to herein as mm H₂O.

Basis weight was determined by ASTM D-3776, which is hereby incorporatedby reference and reported in g/m².

Thickness was determined by ASTM D177-64, which is hereby incorporatedby reference, and is reported in micrometers.

Fiber Diameter was determined as follows. Ten scanning electronmicroscope (SEM) images at 5,000× magnification were taken of eachnanofiber layer sample. The diameter of eleven (11) clearlydistinguishable nanofibers were measured from the photographs andrecorded. Defects were not included (i.e., lumps of nanofibers, polymerdrops, intersections of nanofibers). The average fiber diameter for eachsample was calculated.

Stiffness was measured using a “Handle-o-meter” instrument manufacturedby Thwing Albert Instrument Co. (Philadelphia, Pa.). The Handle-o-metermeasures in grams the resistance that a blade encounters when forcing aspecimen of material into a slot of parallel edges. This is anindication of the stiffness of the material, which has an inverserelationship with the flexibility of the material. The stiffness ismeasured in both the longitudinal direction (machine direction) of thematerial and the transverse direction (cross-machine direction).

Frazier Permeability is a measure of air permeability of porousmaterials and is reported in units of ft³/min/ft². It measures thevolume of air flow through a material at a differential pressure of 0.5inches (12.7 mm) water. An orifice is mounted in a vacuum system torestrict flow of air through sample to a measurable amount. The size ofthe orifice depends on the porosity of the material. Frazierpermeability is measured in units of ft³/min/ft² using a Sherman W.Frazier Co. dual manometer with calibrated orifice, and converted tounits of m³/min/m².

EXAMPLES Example 1

Nanofiber layers were made by electroblowing a solution of nylon 6,6polymer having a density of 1.14 g/cc (available from E. I. du Pont deNemours and Company, Wilmington, Del.) at 24 weight percent in formicacid at 99% purity (available from Kemira Oyj, Helsinki, Finland). Thepolymer and solvent were fed into a solution mix tank, the solutiontransferred into a reservoir and metered through a gear pump to anelectroblowing spin pack having spinning nozzles, as described in PCTPatent Publication No. WO 03/080905. The spin pack was 0.75 meter wideand had 76 spinning nozzles. The pack was at room temperature with thepressure of the solution in the spinning nozzles at 10 bar. Thespinneret was electrically insulated and applied with a voltage of 75kV. Compressed air at a temperature of 44° C. was injected through airnozzles into the spin pack at a rate of 7.5 m³/minute and a pressure of660 mm H₂O. The solution exited the spinning nozzles into air atatmospheric pressure, a relative humidity of 65-70% and a temperature of29° C. The polymer solution throughput of the nanofiber-forming processwas about 2 cm³/min/hole. The fibers formed were laid down 310 mm belowthe exit of the pack onto a porous scrim on top of a porous belt movingat 5-12 m/minute. A vacuum chamber pulling a vacuum of 100-170 mm H₂Obeneath the belt assisted in the laydown of the fibers. A 40 g/m² basisweight spunbond nonwoven material obtained from Kolon Industries (S.Korea) was used as the scrim. The scrim had a stiffness of 35 g in thelongitudinal direction and 55 g in the transverse direction.

The SN structure produced was challenged at various particle sizes forfiltration efficiency using a TSI tester 3160, and the results are givenin Table 1. Efficiencies reported in the data below are for 0.3micrometer particle challenge only.

Example 2

An SN structure was made as described in Example 1, but at a higherbasis weight of the nanofiber layer. The resulting structure waschallenged at various particle sizes for filtration efficiency, and theresults are given in Table 1. TABLE 1 Nanofiber Nanofiber basis PressureFrazier air diameter weight Efficiency Drop permeability Ex. No. (nm)*(g/m²) (%) (mm H₂O) (m³/m²/min) 1 341/387 3 69.9 3.7 37 2 374/362 5 856.4 22*first measurement/second measurement

Example 3

A filtration medium having an SNS structure was formed by handconsisting a nanofiber layer having a basis weight of about 3 g/m²sandwiched between two spunbond layers each having a basis weight ofabout 21 g/m² made from bicomponent sheath-core fibers having a sheathof polyethylene (PE) and a core of poly(ethylene terephthalate) (PET).The average diameter of the nanofibers was about 651 nm. The nanofiberswere nylon. The Frazier air permeability, the pressure drop and theefficiency of the filtration medium are listed in Table 2.

Examples 4-10

Filtration media were formed as in Example 3, with the exception thatthe media of Examples 4-10 had various numbers of nanofiber (N) layersand meltblown (M) layers sandwiched between the two spunbond (S) layers.The meltblown layers were made from side-by-side PET-PE bicomponentfibers, each meltblown layer having a basis weight of about 17 g/m². Thestructure of each medium, basis weight of the nanofiber layer, basisweight of the filtration medium, Frazier air permeability, pressure dropand efficiency of the filtration medium are listed in Table 2.

Examples 11-15

Filtration media were formed by electroblowing layers of nylon 6nanofibers onto a spunbond nonwoven support. The average diameter of thenanofibers was between about 300 and 400 nm. The number of passes toform the nanofiber layer, the nanofiber layer basis weight, the mediaFrazier air permeability, pressure drop and filtration efficiency arelisted in Table 2.

Examples 16-17

Filtration media were formed by electroblowing layers of nylon 6,6nanofibers onto a scrim. The media of Examples 16-17 had SN structuresincluding multiple passes of the scrim layer through the electroblowingprocess. The scrim was a bilayer structure containing a layer of cardednylon and a layer of carded polyester which was subsequently thermallybonded, obtained from HDK Industries, Inc., having a basis weight ofabout 62 g/m². The average diameter of the nanofibers was between about300 and 400 nm. The number of passes to form the nanofiber layer, thenanofiber layer basis weight, and the filtration media basis weight,Frazier air permeability, pressure drop and filtration efficiency arelisted in Table 2.

Example 18

A filtration medium was formed by electroblowing a single layer of nylon6,6 nanofibers onto a moving collection belt in an electroblowingprocess at low vacuum pinning pressure, resulting in a lofty nanofiberlayer. The medium had a nanofiber layer with a basis weight of about 20g/m², formed in a single pass through the electroblowing process. Theaverage diameter of the nanofibers was between about 300 and 400 nm. TheFrazier air permeability, pressure drop and filtration efficiency of thefiltration medium are listed in Table 2. TABLE 2 Frazier air NanofiberMedium perme- Pressure basis basis ability Drop Effi- Ex. Medium weightweight (m³/m²/ (mm ciency No. Structure (g/m²) (g/m²) min) H₂O) (%) 3SNS 3.05 45.7 10.8 3.94 62.76 4 SMNNS 6.1 65.7 4.58 8.17 89.07 5 SMNNNS9.2 68.8 3.41 11.1 94.03 6 SMNNNNS 12.2 71.8 2.87 14.9 97.53 7 SMMNNS6.1 82.7 3.57 11.6 92.48 8 SMMNNNS 9.2 85.7 2.86 15.0 97.22 9 SMMNNNNS12.2 88.8 2.51 17.7 98.14 10 SMMNNNNS- 24.4 178 1.21 35.7 99.93 SMMNNNNS11 SN (1 pass) 5 14.6 3.30 64.50 12 SNN 10 7.31 6.82 87.14 (2 pass) 13SNNN 15 4.57 10.2 91.94 (3 pass) 14 SNNNN 20 3.35 14.0 96.00 (4 pass) 15SNNNNN 25 3.35 16.2 96.99 (5 pass) 16 SNNNN 17.0 79.4 2.74 16.2 99.13 17SN 3.63 66.0 4.57 10.6 95.50 18 N (1 pass) 20 n/a 3.57 10.2 96.40

Comparative Examples 19-26

Filtration media having an SNS structure were formed by hand of nylonnanofiber layers having between about 0.3 and 0.5 g/m² basis weightsandwiched between two scrim layers, each scrim layer having a basisweight of about 17 g/m². In Comparative Examples 19-20, the scrim layerswere spunbond PET on both sides. In Comparative Examples 21-22, thescrim layer was a spunbond PET on one side, and a spunbond nylon(obtained from Cerex Advanced Fabrics) on the other. In ComparativeExamples 23-24, the scrim layer was a spunbond PET on one side, and abilayer structure containing a layer of carded nylon and a layer ofcarded polyester which was subsequently thermally bonded (obtained fromHDK Industries, Inc.) on the other. In Comparative Examples 25-26, thescrim layers were bilayer structures containing a layer of carded nylonand a layer of carded polyester which was subsequently thermally bonded(obtained from HDK Industries, Inc.) on both sides. The nanofiber layerbasis weight, average diameter of the nanofibers, the media Frazier airpermeability, pressure drop and filtration efficency are listed in Table3. TABLE 3 Nano- Nanofiber fiber Medium Pressure basis diam- BasisFrazier air Drop Effi- Comp. weight eter Weight permeability (mm ciencyEx. No. (g/m²) (nm) (g/m²) (m³/m²/min) H₂O) (%) 19 0.3 917 17.66 1991.58 15.05 20 0.5 20.16 90.8 2.30 28.35 21 0.3 947 16.5 173 1.58 14.2022 0.5 956 16.5 77.1 2.70 23.28 23 0.3 852 19.5 178 1.57 8.61 24 0.5 93019.0 86.2 2.33 18.41 25 0.4 1275 36.0 109 1.38 10.63 26 0.3 206 1.258.60

1. A filtration medium comprising at least one nanofiber layer ofcontinuous polymeric fibers having diameters less than about 1000nanometers, each nanofiber layer having a basis weight of at least about2.5 g/m², and at least one scrim layer, wherein the medium has afiltration efficiency of at least about 20% when challenged withparticles having an average diameter of 0.3 μm in air flowing at a facevelocity of 5.33 cm/sec, and a Handle-o-meter stiffness of at leastabout 45 g.
 2. The filtration medium of claim 1, wherein the totalnanofiber layer basis weight is about 25 g/m².
 3. The filtration mediumof claim 1, wherein the nanofiber layer has a thickness of less than 100μm.
 4. The filtration medium of claim 1, wherein the wherein thecontinuous polymeric fibers of the nanofiber layer have diametersbetween about 100 nanometers and about 700 nanometers.
 5. The filtrationmedium of claim 1, wherein the wherein the continuous polymeric fibersof the nanofiber layer have diameters between about 300 nanometers andabout 650 nanometers.
 6. The filtration medium of claim 1, wherein themedium has a filtration efficiency of at least about 30% and up to about99.97% when challenged with particles having an average diameter of 0.3μm in air flowing at a face velocity of 5.33 cm/sec.
 7. The filtrationmedium of claim 1, further comprising a second scrim layer wherein thenanofiber layer is sandwiched between the two scrim layers.
 8. Thefiltration medium of claim 1, wherein the medium is substantiallyelectrically neutral.
 9. The filtration medium of claim 1, wherein thescrim layer is a spunbond nonwoven web or carded nonwoven web.
 10. Thefiltration medium of claim 1, which has an initial pressure drop lessthan about 30 mm H₂O.
 11. The filtration medium of claim 1, which has aninitial pressure drop less than about 24 mm H₂O.
 12. The filtrationmedium of claim 1, which has a Frazier air permeability of at leastabout 0.91 m³/min/m².
 13. The filtration medium of claim 1, which has aFrazier air permeability of between about 0.91 m³/min/m² and about 48m³/min/m².
 14. The filtration medium of claim 1, wherein the filtrationmedium is pleated.
 15. A process for filtering particulate matter froman air stream comprising passing an air stream containing particulatematter through a filtration medium comprising at least one nanofiberlayer of continuous polymeric fibers and at least one scrim layer,wherein the continuous polymeric fibers of the nanofiber layer havediameters less than about 1000 nanometers and wherein each nanofiberlayer has a basis weight of at least about 2.5 g/m² and a thickness ofless than about 100 μm, and wherein the filtration medium has aHandle-o-meter stiffness of at least about 45 g, and filtering up toabout 99.97% of particles having an average diameter of 0.3 μm in an airstream moving at a face velocity of 5.33 cm/sec.
 16. The process ofclaim 15, wherein at least 20% of particles 0.3 μm and larger arefiltered.
 17. The process of claim 15, wherein the initial pressure dropof the filtration medium is less than about 30 mm H₂O.
 18. The processof claim 15, wherein the total basis weight of the nanofiber layer isabout 25 g/m².
 19. A process of forming a filtration medium comprisingproviding at least one scrim layer having a Handle-o-meter stiffness ofat least about 10 g on a moving collection belt, and depositingnanofibers on the scrim layer to form a single nanofiber layer having abasis weight of at least about 2.5 g/m² to form a filtration mediumhaving a Handle-o-meter stiffness of at least about 45 g and a pressuredrop of less than about 30 mm H₂O.
 20. The process of claim 19, whereinthe total nanofiber layer basis weight is about 25 g/m².
 21. The processof claim 20, wherein the nanofiber layer is formed in a single pass ofthe scrim layer on the moving collection belt.
 22. The process of claim19, wherein the nanofibers are formed by electroblowing a polymersolution from a series of spinneret holes at a rate of at least about 1cm³/min/hole and the collection belt moves at a rate of at least 5m/minute.
 23. The process of claim 22, wherein the polymer solution iselectroblown from a series of spinneret holes at a rate of at least 2cm³/min/hole.
 24. The process of claim 19, further comprising pleatingthe filtration medium.
 25. The process of claim 19, further comprisingadhering at least a second scrim layer onto said nanofiber layer. 26.The process of claim 19, wherein said scrim layer has a Handle-o-meterstiffness of at least about 45 g.
 27. The process of claim 25, whereinsaid scrim layers have a combined Handle-o-meter stiffness of at leastabout 45 g.